Core drilling is used in the investigation of rock formations, in respect of prospecting and in also in respect of many other applications, wherein the formation to be investigated is penetrated with a tubular drill which cuts a circular core from surrounding material, whereafter the core is removed from the borehole for examination. The drill normally used consists of a tubular casing string which has at its face end a drill bit of similar tubular configuration. The drill is driven into the formation by a drilling machine which rotates the casing string while forcing the string into the formation at the same time. The drill bit used will have properties that are appropriate with regard to the properties of the rock formation, although the drill bit will normally consist of a tubular steel shaft that has provided at its face end a matrix that contains hard cutting or grinding elements consisting of diamond, hardmetal or similar material. When drilling in hard rock species there is normally used a diamond bit, so as to obtain a drill crown of sufficient wear strength and length of life. The matrix consists of metal powder which has been sintered to an homogenous tubular configuration which is held intact by the abrasive particles.
Diamond-equipped drill bits are normally divided into two types, surface-inset and impregnated bits respectively. Surface-inset bits have a number of diamond crystals in the matrix surface layer and the drill is considered to be worn out when these crystals have been worn down. In the case of impregnated bits, on the other hand, the matrix powder is mixed with a large number of small diamond crystals and as the matrix becomes worn fresh diamond crystals are constantly exposed until the entire matrix has been worn away. The length of life of this latter bit is thus much longer than the former bit.
A large amount of heat is generated in the drilling operation, due to the friction acting between the matrix and the rock, and it is necessary to cool the bit constantly in order to prevent its destruction. The coolant used in this regard is normally water which is pumped through the casing string right up to the drill matrix then either returns to the borehole opening through the space defined between the wall of the borehole and the outside of the casing, or dissipates through cracks and the like in the drilled formation.
In addition to cooling the drill bit, the water is also intended to carry away sludge and slime, e.g. the crushed rock, formed in the drilling operation. These two purposes require the supply of large volumes of water, the amount required depending on the diameter on the drill bit. The gap present between the face of the bit and the rock is, of course, very small, almost nonexistent, and in order to ensure that sufficient water is delivered, the bit is provided with radially through-penetrating water-delivery slots. In order to remain functional during the entire length of life of the bit, it is necessary that these slots are equally as deep as the height of the matrix.
Core drilling is used for borehole depths of from a few meters down to a thousand meters or more. The casing string consists of a number of tubes that are screwed together as the depth of the borehole increases. Each tube will have an individual length of between 1 and 6 meters. During the drilling operation, the core is lifted up in a length that can vary from 1 meter to 6 meters or 9 meters. In the case of conventional drilling operations it is necessary to lift the entire casing string from the borehole, which in the case of deep holes takes a significant length of time, since each individual casing must be unscrewed, lifted away and then screwed together once more. The wire-line technique has been developed because of this. This development involves the use of a special catching device which is lowered by a hoist inside the casing string and grips an inner core tube that firmly holds the core and therewith enables the core to be hoisted from the borehole. This method thus enables the casing string to be kept in the hole until drilling is complete or until the drill bit is worn out, i.e. until the matrix has been consumed. It is necessary to remove the casing string from the hole in order to replace the drill bit.
It is thus highly desirous in the case of wire-line drilling that the bit has the longest possible length of life. Perhaps the most obvious way of increasing the life time of a bit is to increase the height of the matrix and at present matrix heights of up to 12 mm are used with this in mind. However, when the height is increased above this magnitude, a number of drawbacks occur.
Cooling of the bit face is effected with the aid of through-penetrating slots that extend radially in the matrix and transversely through the matrix material and through the full height of the matrix right up to the rock abutting surface of the bit face. As the height of the matrix becomes greater, the water delivering slots become deeper, wherewith a major part of the cooling and flushing water passes through the water delivery slots without reaching the cutting surface, therewith impairing cooling of the bit and also the danger of overheating, i.e. melting of the matrix increases. This quickly leads to wear.
There is also obtained a certain degree of conicity of the inner diameter due to wear, wherewith a core cut from the material during a drilling operation is liable to fasten in the drill bit when attempting to hoist up the core.
Moreover, the increasing height-width-ratio formed by the deep water-delivering slots makes the matrix segments more liable to bend, wherewith segment breakages may occur during a drilling operation.
It is earlier known from Russian patent specification SU1086112 to provide a drill bit with external and internal coolant conveying slots. However, these slots are wedge-shaped so as to create cuffing edges which result in high flushing pressures and in burning of the bit in the case of high-speed drilling in hard crystalline rock, due to an excessively low degree of cooling. The flushing holes also taper, so that the flushing effect and the degree of cooling decrease when the drill bit wears down, this being, inter alia, a problem that the present invention is intended to solve.
Furthermore, the drill face according to the Russian specification has the form of a wedge through which slots are formed in the wedge apex to the extent that the external and the internal slots extend radially beyond the wedge apex. The height of the through-penetrating part is also very small, meaning that the through-penetrating slots will have disappeared, when the matrix has worn down by only 10 percent. The geometry of the drill bit is thus quite different from the geometry of the inventive drill bit.